Apparatuses for metering two or more flowable reaction components are used for the preparation of reaction mixtures for filling cavities, and in particular for filling the cavities of molding tools. The moldings produced in this way, such as, for example, shock absorbers or front parts of motor vehicles, and dashboards, must meet stringent requirements. One of the most important conditions therefor is to combine the reaction components in the correct mixing ratio with the minimum deviation therefrom.
Suitable apparatuses which operate, for example, with piston pumps or gear pumps are generally used for this purpose. They are used in high pressure applications in conjunction with so-called injection mixers and in low pressure applications in conjunction with stirring mixers and/or static mixers.
High pressure metering apparatuses which are rigidly fixed directly to the molding tool, optionally via ancillary mixer heads, are preferably used for handling highly reactive reaction systems having setting times below 5 seconds.
Fast-running series, axial or radial piston pumps are generally used as metering pumps. Such pumps having, for example, 6-, 7- or 10-cylinders, are operated at speeds ranging from about 900 to 1500 r.p.m. at metering pressures of up to 250 bar and higher. High piston speeds and the tolerances between pistons and cylinders as well as tolerances in the control members of the pumps limit the viscosity of the components to be metered to a maximum of about 3,000 mPas. Metering of filler-containing components is not generally possible due to their abrasive behavior. With the conventional individual drive for such pumps by means of squirrel-cage induction motors, changes of pressure in the metering system inevitably lead to changes of speed and thus to an undesirable influence on the metering output. Other disadvantages of such rapidly running piston pumps in the production of moldings include problems of leakage between the metering pistons and cylinders as well as leakage through the additional control members between the suction side and delivery side of the pumps, high disturbance through noise, and the pulsed nature of the metered stream produced because of the delivery characteristics of the pumps.
It is also known to use apparatuses with displacement pistons as high pressure metering apparatus. Using this type of equipment, it is possible to process highly reactive and optionally filler-containing or high viscosity two-component systems in a strictly prescribed metering ratio which generally corresponds to the mixing ratio. The metering units are driven in this case by a hydraulic piston and cylinder unit rigidly connected to the metering units. This machinery allows the metering ratio to be changed (as is necessary when the formulation or filler is altered) in relatively large steps. Although it is possible to make a freely selected modification in the metering ratio during a metering stroke, the adjustment means operates very slowly and depends upon the predetermined large steps.
Equipment having two or more displacement piston pumps which may be driven by means of a common rotatable yoke are also known. These types of machinery do in fact allow the metering ratio to be modified more or less continuously. They are also used as low pressure instruments at a metering system pressure below 20 bar during the processing of slowly reacting systems. Such types of apparatus are not generally used for the high pressure metering of highly reactive systems at pressures above 200 bar because such a design requires a very high mechanical outlay at a filling volume above about 3 liters, due to the high actuating forces necessary. Another disadvantage lies in the change in position of the metering systems for the individual reaction components in relation to each other when the metering ratio is changed because the distance to the center of motion of the yoke changes in the process. Rigid coupling of the metering units and thus rigid piping or integration of all high pressure pipes leading to the mixer in a common housing, required for reasons of safety during high pressure processing, is not possible. Both systems have the particular disadvantage that each metering unit must be provided with a pressure limiting element (for example, a bursting disc or a spring-loaded ball valve) for reasons of safety, since the driving forces required for the two metering units could overload one of the systems if the other were to break down.
This disadvantage does not arise with apparatuses having individual drive for each metering unit since the transmission ratio between the hydraulic drive piston and the metering piston may be designed to be such that overloading caused by a maximum allowable pressure being exceeded in the high pressure metering pipes leading to the mixer head is not possible.
Thus, apparatuses have already been proposed in which the metering ratio is regulated by electronically controlled electro-hydraulic pilot valves. These types of apparatus consist of a displacement piston pump with a hydraulic drive cylinder and pilot valve flanged thereon for each component to be metered. The movement of a "leading" metering piston is sensed in this case. A movement of one or more "following" metering pistons corresponding to the metering ratio is electronically preselected and regulated via associated electro-hydraulic pilot valves. The movement of the "following" metering pistons is thus controlled on a master-slave basis. However, with this apparatus, particularly in the case of high advancing speeds for the metering pistons, constancy in the metering piston speeds in the preselected ratio required during the entire metering stroke and at any moment in the metering stroke is not possible to a tolerance of less than 1%. In addition, variations in pressure increase the tolerances both in the hydraulic drive system and in the metering system for the flowable reaction components which are formed, for example, during the opening and closing process for the associated mixing heads. This is caused by the unavoidable oscillation of hydraulic systems which are controlled by pilot valves. The accuracy of metering is also adversely affected by temperature-dependent variations in the viscosity of the hydraulic oil.
Typical apparatuses of the types described above aare disclosed in U.S. Pat. No. 3,901,408, U.S. Pat. No. 3,908,862, U.S. Pat. No. 4,008,829, U.S. Pat. No. 4,030,637, Great Britain Pat. No. 1,405,060 and Great Britain Pat. No. 1,535,968 and/or manufactured by Binks Manufacturing Co., Franklin Park, Ill.; Cincinnati Milacron Inc., Cincinnati, Ohio; Desma-Werke GMBH, D-2807 Achim, Federal Republic of Germany; Pyles Industries Inc., Warren, Mich.
An object of the invention is to provide an apparatus which ensures exact volumetric metering of flowable multi-component reaction systems into a mixing chamber, and more specifically in freely selectable and exactly reproducible metering ratios, wherein the deviation in the predetermined value of the metering ratio is to lie below 0.5% at any moment in the metering stroke, independently of external influences such as, for example, variations in pressure in the hydraulic and/or metering system.